The present invention relates to a patterning method and in particular to a patterning method for fabricating electronic or optical devices, such as light emissive devices, integrated circuits and optical filters, incorporating organic polymer materials.
The present invention also relates to electrical and optical devices fabricated by the patterning method.
In recent years, display devices incorporating electroluminescent organic polymer light emissive materials have been proposed and these are frequently referred to as electroluminsecent (EL) displays.
In such displays, a soluble electroluminescent polymer is deposited onto a solid substrate such as, for example, a glass, plastics or silicon substrate.
Spin-coating is the typical deposition method for the polymer EL display. This does not need vacuum processes to deposit organic layers, so it may be considered as an inexpensive process. As such only one type of polymer material may be deposited for a device, which limits the use of the materials to a monochrome display. Multicolour displays cannot in practice be achieved without severe fabrication difficulties using a spin coating technique because the various areas of the display would be required to be masked during the spin coat deposition for each required colour. Furthermore the spin-coated polymer layer covers all areas of a substrate involving even areas which are not required to be coated. A plasma etching or laser ablation process is required in order to remove the polymer layer from such areas on the substrate, increasing fabrication costs.
Ink jet printing techniques have been proposed to deposit soluble polymers because of the ability to use inkjet technology for the processing of multi-colour devices and also because of the relatively low cost of using such techniques. It is possible to deposit polymers only on necessary parts, therefore, the removal process is not needed.
It has been realized with the present invention that inkjet technology is also ideally suited to the deposition of the above soluble organic polymer materials to provide not only light emissive displays, but also other types of devices incorporating the organic polymer materials. However, to facilitate the deposition of such soluble materials it is usual for the receiving substrate to be provided with a pattern of wall structures defined in a de-wetting material so as to provide an array of wells or trenches, bounded by the wall structures, to receive the soluble polymer materials being deposited. These wall structures are known within this art as bank structures and this term will be used in this disclosure when referring to such structures.
FIG. 1 shows a cross-section of a part of a light emissive display device. A substrate 2 supports a conductive electrode 4 such as a layer of indium tin oxide (ITO), which acts as the anode electrode for the display device. A bank structure (separator) 6 is provided on the electrode 4 and a light emissive polymer material 8 is deposited into wells defined by the bank structure and into contact with the electrode 4 by using an inkjet technique. The bank structure has a repelling (dewetting) surface, resulting in the deposited material being confined by the walls defined in the bank structure 6. A second electrode 10, which acts as a cathode electrode for the device, is then deposited over the bank structure and into contact with the light emissive polymer material. When a voltage is applied between the anode and cathode electrodes 4, 10, a current flows through the material 8 which, in response, emits light to display an image of a predefined shape or pattern.
As mentioned above, the function of the bank structure is to define the position of the light emitter correctly even when there is some fluctuation in the position of the inkjet deposition. This function is important especially when a number of soluble organic polymers are deposited as an array of dots which are addressed via active or passive matrix addressing schemes because accurate positioning of the dots is necessary for such a array. With displays of the above type any image may be displayed by applying appropriate addressing signals to the electrodes of the matrix addressing scheme, whereby pixels constituted by the dots of organic polymer material are driven to emit light and provide the pixel images of the display. Such displays are able to display either static or moving images.
However, there is also a significant requirement for what are frequently referred to as static displays in which it is required to display a fixed or static image which is defined by physical features of the display. Such static displays may take many forms and can be used for providing information of many kinds to a viewer, such as warning signs, advertising slogans or messages, or information such as displayed on motor vehicle or aircraft panel displays or the like. These static displays may also be provided in combination with the above referred to addressed pixel type displays. In the case of a static display, the bank structure 6 serves an important additional function as it is required also to act as an insulator layer so that a short circuit does not occur between the anode and cathode electrodes. The bank structure may comprise any suitable insulator material, such as polyimide, and may be deposited by any suitable process, such as spin coating. However, the bank material, in its deposited condition, comprises a continuous layer and hence forms an insulating barrier to the underlying anode electrode 4. Wells 12 must therefore be defined in and extending through the bank material, to provide the bank structure shown in FIG. 1, thereby enabling the light emissive polymer material to be deposited into contact with the anode electrode which underlies the bank material.
The bank structure is usually defined in the bank material using a photolithographic technique but this is a relatively expensive process, involving an exposure step with photolithographic masks, a developing step, an etching step, stripping and cleaning steps. The static display is a relatively low-price product and the bank structure is not therefore ideal for such a display in terms of a production cost. Furthermore, if the light emissive polymers were to be deposited with an inkjet head in the wells which are defined by the bank structure, the position of the inkjet head needs to be aligned accurately with the position of the wells, resulting in a low throughput and an increase in the fabrication. If there are failures in the alignment, a well in the bank structure may not receive a droplet from the ink jet head, leaving the underlying anode electrode exposed. The subsequently deposited cathode electrode can therefore contact directly the exposed area of anode electrode and the devices do not function correctly due to leakage currents or short circuits between the electrodes.
A static display device fabricated in accordance with the above technique may typically be as shown in FIG. 2, where it can be seen that the anode electrode 4 may comprise conductive patches of ITO, and the bank structure is defined so as to provide letter or symbol patterns of the conductive light emissive polymer 8. The patterns can be changed flexibly because the bank structure is patterned by photolithography. Inkjet printing is, by virtue of its origin, a very flexible printing method, and one can easily change patterns by computer control. However, the bank structure negates such flexibility in inkjet printing techniques.